So… they’re cool looking, right off the bat – it goes with the unapologetic comic-booky-ness of the whole production that the aliens (or whatever they are) would be guys with impossible-to-look-anything-other-than-evil monster-faces; and I loooooove that they seem to use melee weapons, implying LOTS of hand-to-hand combat.
As for what they “are?” Well, it’s entirely possible that they’re something entirely new or just “The Aliens.” But because it’s fun to keep playing the “spot the Marvel Universe reference” game…
The handsome guy on the right is Beta Ray Bill; probably the most significant character in the “Thor” universe outside of Loki and Thor himself. Created by Walt Simonson in his landmark Thor run in the 80s, Bill is a the sworn protector of The Korbonites, a nomadic/planetarily-displaced alien race whose homeworld was destroyed by Thor villain Surtur.
You may notice that he dresses like Thor. Why? Well, when Thor first encountered Bill they mistoke eachother for enemies. During the fight, Bill got hold of Mjolnir (Thor’s magic hammer) and – per the rules of Odin’s “If Ye Be Worthy, Possess The Power of Thor” spell – was imbued with Thor’s powers. This was kind of a big deal at the time, because very few other beings in the Marvel Universe are supposed to be “worthy” enough to lift it without some kind of loophole or an exception from Odin (Captain America can do it, of course.) Eventually, Bill got his own equivalent to Mjolnir called “Stormbreaker,” and he’s been a fan-favorite Marvel mainstay ever since.
Now, I doubt that that’s Bill in the “Avengers” screencap, but given that A.) Loki as the main heavy and The Cosmic Cube as the likely main MacGuffin tie this very close to the “Thor” universe to begin with and B.) “Thor 2” is further into pre-production than any post-“Avengers” Marvel sequel other than “Iron Man 3;” these guys could very well be Korbonites (controlled/enslaved by Loki?)… and if one of them were to, say, snatch up Mjolnir and suddenly sprout Asgardian armor it’d be one HELL of a tease for “Thor 2” – to say nothing of a garaunteed tear-the-roof-off moment of fanservice.
On the other hand, The Korbonites don’t all look like Bill (he was “modified” into a more monstrous form as part of making him a better soldier) in the comics, but that doesn’t necessarily need to be the case here. Definitely a longshot either way, but… honestly? How cool is it that something like that isn’t a complete impossibility?